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Insulating Gap Between Exterior and Interior Foam

jwalker1511 | Posted in GBA Pro Help on

Hi all, I have an insulation question. My house was planned as a slab inside 5′ frost walls (zone 6 southern coastal maine). After walls were poured the builder at the time and I decided to change it to a crawlspace with a framed floor instead, due to a couple of problems that came up. So the architect designed a floor framed on 2×12 PT ledgers attached to the inside top foot of the frost wall with wedge anchors, so the subfloor sits at the level the slab was supposed to be. I insulated the crawlspace with 3 1/2″ XPS inside the walls from top of footing up to about an inch below where the ledger would go, taped seams, and before that Drylok’d interior and exterior everywhere there wasn’t going to be foam board. Also have an exterior dimple mat on lower 4′ of walls.  The house will also have continuous exterior insulation, mostly as 3 1/2″ EPS EIFS system for siding, with some areas having a band of faux stone at the bottom with 3″ graphite EPS under it. Grade is at about 6-10″ below top of frost wall, depending if it’s near garage doors or other parts of the house. So that leaves me with several inches of space to figure out how to insulate, between the bottom of exterior foam/siding and the top of the XPS on the interior. My 2×12 ledger area therefore has the top part of it insulated on the exterior with foam board, but the lower part of it has just cold concrete behind it. I put foam sill sealer behind the PT ledgers so they aren’t touching concrete. But now I am not sure if I should closed cell spray foam the inside of crawlspace walls from the top of the interior XPS up to the bottom of the subfloor, encapsulating the ledger? This will create a foam sandwich for the top half of my 2×12 ledger. It can’t dry to the exterior through the concrete anyway but it would eliminate it being able to dry to the interior. Or use rockwool insulation instead? This would allow it to dry but also allow interior air to reach the cold concrete wall on the bottom half of the ledger. With none of the exterior insulation up yet I’ve already witnessed tons of condensation along the ledger and joist hangers just from hot humid days. So I’m leaning towards the spray foam. Thoughts?

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  1. Expert Member
    BILL WICHERS | | #1

    Don't use rockwool (mineral wool) on the interior -- inside air will get through and you can have condensation on the inside of the rim joist behind the mineral wool. You don't want condensation, that will keep the rim joist wet, which is obviously a problem. Can you extend the exterior foam to cover down bast the lower edge of the rim joist? That would be my preferred solution if possible. If you can't do that, you can foam in EPS panels against the rim joist (which is what I did in my own home), since EPS is at least a little bit vapor permeable. I used 2" EPS on my own home, since I have foil faced polyiso on the exterior, which means no drying to the exterior. The downside is it's pretty labor intensive to insulate the rim joist this way.

    If you can't do either of the above, then closed cell spray foam is probably your remaining option. If you use spray foam, you want to encapsulate the interior side of the rimjoist from the subfloor all the way to the interior rigid foam on the foundation wall. Check if you need to leave a termite inspection gap in your area though -- if you do, you normally leave that gap in the upper few inches of the masonry.

    Your rim joist will stall have some drying through the "exposed half" on the exterior if you spray foam the interior side.


  2. Expert Member
    Michael Maines | | #2

    What does your architect suggest?

    In Maine, vapor drive is almost always from interior to exterior, so your assembly would only dry to the interior if it was quite wet. Insulating with mineral wool without a vapor retarder would promote moisture accumulation at the ledger. Pressure treated lumber is rot-resistant but not rot-proof; it's best to keep it dry if possible since it's not something like a deck that you can easily replace in a few decades.

    Sill sealer is a decent capillary break but it doesn't provide much insulation.

    As you've probably figured out by now, the best way to handle this situation is by installing foam behind the ledger, either by creating a shelf in the concrete wall, using a pony wall under the ledger or using a metal angle to support the ledger. But you're beyond that point now.

    Unless your concrete is an unusual mix, it will allow drying to the exterior. Moisture flow doesn't follow the straight lines on drawings, so as long as the concrete is exposed to open air above grade, I think your best bet at this point is to insulate the interior of the rim joist with closed-cell foam. Or use a thin layer of closed cell as a flash coat and mineral wool batts to beef up the R-value.

    1. jwalker1511 | | #3

      Thanks Bill and Michael. There was a 6" x 2" ledger poured in the top of the concrete wall for the original slab insulation which I did fill in with 2" XPS behind the 2x12 before we installed it. So the top half of the 2x12 has insulation behind it from that and the exterior foam under the siding. But the bottom of the 2x12 extends a few inches below that point and is on the bare concrete. Wedge anchors are located there. That part mostly falls just below grade. The builder I had last fall who did the foundation/groundwork suggested I put all the frost wall insulation on the interior so I did, and it has already been backfilled so there's not much option of running the exterior foam below grade without a lot of labor. I think there will be a little bit of concrete exposed to the air, I was going to try to run the faux stone pretty close to grade and put down crushed stone around the perimeter so it would look nicer. The EIFS on the back of the house will probably leave more concrete exposed.

      Michael - your comment brought up another question if I can pick your brain - I will have the EIFS exterior and 2x6 walls will be filled with DP cellulose. I was originally going to use Siga Majrex for an interior vapor barrier but was told I should do away with that and have a vapor open interior with just painted drywall (Unvented cathedral ceilings are to be 5 1/2" CC spray foam and rest of 2x12 cavity DP cellulose, possibly tongue and groove wood ceiling). But if most vapor drive in Maine is from int to ext, should I do more to prevent interior moisture from camping out in my cellulose walls & ceiling? Majrex or no Majrex or something else?

      1. Expert Member
        Michael Maines | | #4

        I understand now. Have you checked the manufacturer specs on the stone? They probably have a minimum height above grade for their warranty. I wouldn't exceed that. You have a fairly unique situation so it's hard to advise you confidently. I still think I'd do spray foam at the interior, with or without mineral wool. 2" XPS allows a little drying and the stone might allow some drying as well, especially in cold weather when the vapor drive is strongest. It's not the most resilient assembly but if the ledger is sealed from exposure to air, I think you will probably be ok.

        3 1/2" EPS is about R-13-14 so if your cavity insulation is R-20ish cellulose, you meet the code requirement for eliminating the vapor retarding membrane, as long as you have latex paint on drywall at the interior. But EIFS is easy to get wrong; if it was my project, I would use Siga Majrex (or another variable permeance membrane) to be safe--there's not a downside other than cost. I would also recommend a drainable WRB such as Tyvek Drainwrap on the exterior, between the sheathing and EPS, to drain any accumulated water. But I'm cautious because I have only heard horror stories about EIFS failing from improper installation.

        1. jwalker1511 | | #5

          I emailed the faux stone company (its a hard polyurethane product) asking that and their response was whatever your local building codes say. So I figured with gutters and crushed stone at the base hopefully the splash back wouldn't be too bad. Cultured stone is too costly and this product I could do myself and is light enough where I feel more comfortable attaching it over my 3" of G-EPS. I will put a drain mat behind it for air circulation too.

          The EIFS contractor uses Sto's line, they have their own liquid roll on WRB and air/water sealing products that work with their glue. I will probably be the one doing all the flashing/weatherproofing details before they do the WRB and EIFS. The glue is supposed to create a tiny space for a drainage gap, I was concerned about it so he said he'd put on an extra thick bead. I plan on making sure they leave a venting pathway at top and bottom of wall. Hopefully it will all hold up!! Thanks for your help!

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